Noise reduction system



March 21, 1944- lla'. L HOLLINGSWORTH 2,344,597 f NOI-SE REDUCTION SYSTEM FiledApril 17, 1942 2 sheets-sheet i *Tlcrlcn INVENTOR j! f?. Les f/oLL/Nsn/oerfn ATTORNEY Mmh 2, 1944.

R. l.. HoLLqNGswoRl-H 2,344,697

NOISE REDUCTION SYSTEM Filed April' 17, 194.2 v

- T'lclib 2I sheets-sheet 2 4/Vo/.ss 1mm/LSE y -W-W y B T1-ab.

J\ fm1-EC- 1 .Ed T INVENTOR -BY #5K/WW ATTORN EY Patented Mar. 21, 1944 R. Lee Hollingsworth, Riverhead, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application April 17, 1952, Serial No. 439,363

(Cl. Z50- 20) 7 oiaims.

My present invention relates to a circuit for the reduction of man-made noise 'and static, as well as atmospheric static of certain kinds such as sharp clips and short clashes.

Man-made static such as ignition, commutator ripple, etc., are often several times as strong 'as the desired signal, whether it be the carrier or modulation level which one wishes to listen to, or use. These short, high-amplitude impulses represent a considerable amount Vof energy, and are capable of actuating a reproducing unit to such an extent as to make listening undesirable. Likewise in radio-telegraph reception, when these strong impulses are passed directly into a narrow band filter, they set up a ringing in the circuit beyond the filter, asa result of overloading i the lter, which lengthens the duration ofthese clicks and short static crashes. If these impulses are instantaneously reduced, or eliminated entirely, they become tolerable through a reproducing unit (loud speaker) or are discriminated against by the aforesaid narrow band filter in radio telegraphy because of non-overloading.

An important object of my invention is to pro-- vide a method of controlling high amplitude noise and static components of short duration; the method including the steps vof rectifying "received signal energy, grid-controlling the rectification from an auxiliary rectifier which becomes operable to high level components, and reduces, or cuts off completely, the output of the signal rectification instantaneously during reception of high level noise impulses.

Another object of the invention is to provide a noise impulse rectifier for controlling the signal demodulator of a receiver; an auxiliary rectifier being provided to rectify solely yamplified modulation signals, the output of the auxiliary rectifier functioning through a time delay circuit to control the noise rectifier sensitivity as a function of the modulation signal level.

The novel features which I believe to `be characteristic of my invention are set forth in particularity in the appended claims; the invention itself, however, as to both itsorganization and method of operation Vwill best be understood by reference to the following description taken in connection with the drawings in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into effect.

In the drawings:

Fig. la shows a receiver embodying the invention,

Fig. 11b shows fa modification,

Fig. 2a to 2d inclusive illustrate the `functioning of the noise control system.

Referring now to Fig. la, the modulated carriei wave collector I is coupled to radio frequency amplifier 2. The collector I may be a 'grounded antenna system, as well as the dipole shown. The amplifier may comprise one or more tunable amplifiers; the tuning range may be in any of the well known bands. The amplified energy, which may comprise modulated carrier wave energy having noise and static impulses superposed thereon, is passed to a mixer 3. The Alatter may be of the combined first detector-local oscillator type, or the first detector may be independent of the oscillator. Since superheterodyne practice is universally followed, thattype Vof reception is shown. Of course, any other type lof reception may be used. The intermediate frequency (I. output of the mixer is transmitted through the I. F. amplifier 3. The output circuit 3 vof the amplifier is resonated to the operating I. F. value, and feeds each of resonant circuits 4 and f5 as shown.

Each of circuits 4 and 5 is tuned to the operating I. F. value; they serve as input circuits foi respective rectifier tubes 4 Vand 5. Tube 4 is a grid-controlled diode so far as the applied signal vcurrents are concerned. Physically the tube i 'is `a triode whose anode is connected to the high potential side of circuit 4. The low potential side of the latter is connected to ground through load resistor 6. Modulation signal voltage, whether voice or music, is transmitted through condenser I5 to transformer I6. The latter 1may feed any desired type of audio frequency output circuit.

.Direct current voltage is taken off from resistor 6, and is applied to the control vgrid 'I' of tube 1. The plate of the latter is connected to the positive terminal of direct current lsource 8 in series to ground Withresistor 9. Cathode 8 is at ground potential, and, therefore, the tube I is normally conductive in the absence of received signal energy and consequent absence of negative bias `across resistor B. Hence, in the absence of received signal energy the voltage across resistor 9 is a maximum. The latter voltage, negative `to ground, is taken off by tap 9', and is applied to the signal Agrids of the controlled tubes rover a path comprising resistor I0, connection II and the various filter resistors I I". 'I'he various tubes of the amplifier 2, mixer 3 and I. F. amplifier 3' may be gain-controlled. Hence, during interstation tuning, or when the received carrier strength is vtoo '-low 'or lduring selective vfading may be augmented by the effect plied over the closed switch I9.

periods, the bias derived from resistor S reduces the gain of the receiver sufciently to prevent reproduction of undesired noises.

When carrier-derived voltage is developed across resistor 6, the grid l' is biased to cut-off thereby reducing the voltage across resistor 9 to Zero. The AVC bias developed across resistor 6 is now fed through resistor Ii, which latter resistor is in'series with resistor I to leadrv I-If. Condenser I8 is connected from the junction oi' resistors II and ill to ground. Network II-Iil- I8 provides proper time delay action for satis# i" .tionwas interfered with by man-made static factory operation of the AVC circuit. Rectifier diode 5 acts to draw current when the noise peaks, or signal peaks, reach above fa.p're'deter-'" ,due to source I3, there is developeda negative vvoltage across resistor i2 which is app-hed tothe ,grid of rectifier 4 to reduce the .outputthereof instantaneously, or to block it entirely, for the duration ofthe noise click or apparent overwould start to ilow through the filter, and, once started, could not change suddenly enough to drop out. Perhaps, it would dip slightly, and in most cases, if the click was short enough in duration, would not be noticeable past the lter as shown at D in Fig. 2d. If the modulation consisted of voice or music, if the click were lowered to the level of the modulation or taken out completely, it' would be somewhat less objectionable.'

Now let it be assumed that it was desired to v'listen to a program on short waves, and recepe such as motor starters and commutation, auto .ignition fromithefstreet or nearby locations. If

i the noise suppressing circuit operated only above the highest'modulation level, then when the f modulation was low, or no modulation was used modulated components that reach a very highl vlevel during certain types of fading. Naturally, 4if the desired modulation drew current through diode 5 the quality would suffer due to the demodulating eiect that would take place in tube 4. Diode 5 could be made to operate, for example at 35 to 60 decibels, above rectifier l by reducing .the coupling .between circuits 3 and 5suficient- .1y to separate the current starting po-ints between diodes 4 and 5. The latter coupling, however, may have the -same coeicient as that between 3 and 4', but

is rendered conductive to higher signal and noise levels only by inserting source I3 in serieswith the plate circuit of tube 5 and/or the'grid cir.-

cuit of tube 4. If additional control is desired, switch I9 maybe inserted in the lead from the lanode end of resistor I2 to the gain control con,-

-neotion- II. lIn this way the effect of the bias from resistor.I2 applied to the grid of tube 4 lof the bias ap- =Explainingthe actionof this circuit further, in

Fig 2a the curve A shows theamplitude of a :rectified carrier, modulated 100% for a time by a tone impulse. A heavy noise click is shown extending much higher in amplitude and containying many components jammed together` to give it the appearance of a square wavecharacter.

-This peaked signal starts to reduceitself auto-.

matically as it exceeds the tone modulation level;

this is due to the action of diode-5 on vthe -grid 4of, rectifier 4. In Fig. 2b, the curveB shows the noise click reduced to the approximate level of the modulation. By increasing. thevoltage control of rectier 5, or by applying the control voltage to more tubes in the receiver, the4 de in.Fig. 2c.

Let us assume for explanatory purposes that this tone was thenipassed throughs narrow lband lter capable of responding only to impulses not .faster on shorter in durationthan the impulse containing the ve cycles of thetone. TheV tone 4lation component completely.

at all, we would have the noise of a high order prevailing if it were not high enough to knock itself down, or out, completely. This condition .can be corrected by causing the sensitivity of the noise rectifier to vary as a function of the modulation level.

Referring to-Fig. 1b, let it be assumed that Athe tube 5" is a triode which for signal currents acts as a grid-controlled diode. The grid of tube 5`is connected through a time delay circuit to a rectifier that rectii'les a portion of the modulation,v after it has been amplied. Then, as the vInusicdecreases in volume the reduced rectified modulation voltage would allow the sensitivity of 'tube 5" to be increased, and the noise that was 'normally above this low modulation level would be lowered'in'amplitude. Likewise, if the modu- .lation stopped completely the sensitivity of rectifier 5" would be increased to take out most all objectionable noises. In the event of a sudden increase in music level the rst few impulses would be lowered, but not for longer than it would take the increased modulation level again to reduce the sensitivity of tube 5 through the time delay circuit. There maybe strong opposition to a sudden rise in modulation level, but

by choosing the proper time constant values between the grid of tube 5" and the modulation rectier load resistance, sharpimpulses of high amplitude would be reduced considerably in arnplitude before reaching themodulation rectier. However, a series of sustained components,

such as modulation, would lower `the sensitivity AOf tube 5^', sincethe control factor of tube 5f would not be sufficient to knock o -ut each modu- Therefore, any increased level ofsustained modulation (for perhaps a small fraction of a second) would start reducing the sensitivity of tube 5 through the .time delay circuit. If the control of tube 4 by tube 5'? was great enough to demodulate com pletely the modulation as it attempted to rise high very suddenly, then this circuit would be useless in sound broadcast reception. Therefore,

the sensitivity of tube 5" must vary slowly in sensitivity and follow the general average of the modulation level, and never have suiicient controlfover tube 4 to reduce completely any component within the modulation range. When the score ormusic is low in amplitude, as tube 4 draws current on a sudden increase in the music level,l the increase must be also amplified to start "decreasing the sensitivity of tube 4 quickly toa point where the modulation would not be aiected bythe action of tube 5 on the grid of tube 4. The highest sensitivity to which tube 5 might reach at any time would -be perhaps from 5 to 15 decibels below the sensitivity of the current starting point of tube 4. This would insure tube 4 to be always the main signal control source, as the modulation level starts from zero, or rises from a low level passage to a higher level passage.

While I have indicated and described several systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be made Without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:

1. A method of receiving modulated carrier Wave energy which includes detecting received energy to provide modulation signals, developing a control voltage in the absence of received carrier energy, utilizing the control voltage to render ineiilcient the supply of electrical energy for detection, deriving a second control voltage from detected wave energy, controlling the level of Wave energy subjected to detection With said second control voltage to maintain uniform said supply of energy, deriving a third control voltage from received energy Whose amplitude exceeds a predetermined level, and utilizing the third control voltage to render said detection inefficient.

2. A method of receiving signaling energy which includes amplifying such energy, demodulating the energy, rectifying the amplified energy when the amplitude thereof exceeds a predetermined level thereby to provide a control voltage for preventing the reproduction of undesired noise impulses, rendering said demodulation inefficient in response to said control voltage, amplifying the demodulated energy, rectifying the amplied modulation, and controlling said first mentioned rectification action in response to the rectified modulation voltage.

3. A system for receiving modulated carrier wave energy comprising means for detecting received energy to provide modulation signals, means for developing a control voltage in the absence of received carrier energy, means for utilizing the control voltage to render inefficient an amplier Asupplying electrical energy for detection, means for deriving a second control voltage from detected wave energy, means for regulating said ampliiier gain, -for controlling the level of wave energy subjected to detection, with said second control voltage, means for deriving a third control voltage from received energy whose amplitude exceeds a predetermined level, and means utilizing the third control voltage to render said detection ineiilcient.

Cil

4. A system of receiving signaling energy which includes means amplifying such energy, means demodulating the energy, means rectifying the ampliiied energy when the amplitude thereof exceeds a predetermined level thereby to provide a control voltage for preventing the reproduction of undesired noise impulses, means rendering said demodulation ineiiicient in response to said control voltage, means amplifying the demodulated energy, means rectiiying the amplified modulation, and means for controlling said first mentioned rectification action in response to the rectified modulation voltage.

5. A method of receiving amplitude modulated carrier Wave energy Which includes detecting received carrier energy to provide modulation signais, developing a control voltage in the absence of received carrier energy, amplifying the carrier energy prior to detection, utilizing the control voltage to render inefficient said amplication, deriving a second control voltage from detected Wave energy, controlling the said amplification with said second control voltage, rectiiying received energy whose amplitude exceeds a predetermined peak level, and utilizing the last rectied voltage to render said detection ineiective.

6. A method of receiving signaling energy which includes amplifying such energy, demodulating the energy, rectifying the amplified energy when the amplitude thereof exceeds a predetermined level thereby to provide a control voltage for preventing the reproduction of undesired noise impulses, rendering said demodulation inefficient in response to said control voltage, rectifying the demodulated energy, and controlling said first mentioned rectification action in response to the rectied modulation voltage.

7. A method of receiving modulated carrier Wave energy Which includes detecting received energy to provide modulation signals, developing a control voltage in the absence of received carrier energy, utilizing the control voltage to render ineilicient the supply of electrical energy for detection, deriving a sound control voltage from detected Wave energy, controlling the level of wave energy subjected to detection with said see ond control voltage, deriving a third control voltage from received energy Whose amplitude exceeds a predetermined level, utilizing the third control voltage to render said detection inefficient, rectifying the detected carrier energy to provide a fourth control voltage, and regulating said third voltage derivation with said fourth control voltage.

R. LEE HOLLINGSWORTH. 

